1. Field
The present disclosure relates generally to a device to aid a lapping process of two components, and more particularly, to a device to aid in lapping a spherical washer to another steam turbine component.
2. Description of the Related Art
A conventional steam turbine includes a higher pressure turbine and at least one lower pressure turbine coupled to a single shaft. Steam enters the turbine at the high pressure turbine through a stop valve and a control valve. The thermal energy of the steam is converted to mechanical energy in the higher pressure turbine, and the steam is exhausted to reheaters. In each reheater, the steam is dried, reheated, and superheated prior to its entry into the lower pressure turbine. The superheated steam is routed through the stop valve as it travels from the reheater to the lower pressure turbine. Energy conversion occurs again in the lower pressure turbine as the steam expands into the vacuum of the main condenser.
A spherical washer is part of the stop valve assembly that seals off the steam and helps to align the steam turbine shaft. In the stop valve assembly, the spherical washer seats against the shaft shoulder on the flat side and against a casing bushing on the spherical side, the casing bushing disposed concentrically with and surrounding the shaft. Each time the stop valve is inspected during periodic maintenance, the stop valve is removed from the steam turbine, and the spherical washer is replaced with a new one. In order to ensure that the spherical washer will seal properly, the new washer is lapped against its mating counterpart, the casing bushing, in the valve casing and the shaft shoulder. Lapping two components is a machining process which involves rubbing the two mating surfaces of the two components together with an abrasive, or lapping compound, between them.
Holding the spherical washer from the inside in order to lap its spherical seat to the spherical mating surface of the casing bushing has proven to be difficult. For example, the spherical washer, in its assembled location within the stop valve, is in a fairly confined area. A field technician working within the valve to lap the spherical washer will find the working space cramped and difficult to maneuver. A variety of methods have been used by field technicians to lap the spherical washer. As an example, a block of wood has been used to hold the spherical washer by its inner diameter and rotated by hand as the technician manually applies the lapping compound onto the outer diameter surface of the spherical washer. The drawback to this approach is that it very labor intensive.
Consequently, a tool that allows a technician to firmly hold the spherical washer while the washer is being lapped to the casing bushing within the reheat valve assembly is desired.